This invention relates to a variable volume conditioned air distribution system. It also relates to a controller for controlling the volume flow of conditioned air in a variable volume control system.
In variable volume systems the temperature of the air flowing through a duct is kept constant, and the heating or cooling requirements of the room are met by varying the volume of air flowing through the duct to the room.
Since the duct has a constant diameter, the volume of flow through the duct will be dependent upon the velocity of air flow within the duct.
The volume of air flow through the duct can by controlled by measuring the velocity of air flow and by positioning a valve member within the duct to produce a given flow velocity in response to a sensed temperature in the room.
A velocity spring has been used in control systems of this kind for producing a biasing force which determines the velocity set point for controlling the movable valve member in the air flow duct.
A temperature signal sensed by a thermostat in the room has also been used in combination with the bias provided by the velocity spring for resetting the control set point for the air flow velocity in response to changes in the room air temperature.
A smaller room requires less air flow than a large room for any given temperature condition in the room, and a single controller should be capable of providing, for all room sizes, a well controlled modulation of the volume of air flow to the room for all conditions of operation from zero flow to maximum air flow through the full range of room temperatures.
For a small room, the flow volume might vary from zero flow to a maximum flow volume corresponding to an air flow velocity pressure of 0.1 inch of water column. For a large room the volume of flow required might vary from zero to a maximum flow corresponding to 1.0 inch water column velocity pressure.
Controls constructed to provide a good modulation at the relatively high volume flow rates required for a large room have sometimes not been effective to provide good regulation of flow at the relatively low velocity pressures associated with a small room.
The problem of providing satisfactory control over the full range of volume flows can become particularly difficult when the variable air flow volume system is one which uses system air for powering all of the components of the control mechanism.
In such system powered variable air flow volume systems the maximum pressure available to the room thermostat to move a diaphragm in the controller can be 0.6 inch of water column. The force that can be generated by this pressure and lower pressures on the size of a diaphragm that is convenient to use in a controller mechanism can pose problems in positively positioning structure for resetting the set point of the velocity spring.